Attitude Control Of The Soft Landing's Terminal Phase For Lunar Lander

This paper mainly deals with the attitude control problem of the soft landing's terminal phase for the lunar lander. At that moment, the horizontal velocity of the lander is zero, the vertical velocity is low and the deviation of attitude angle is large. The destination of the attitude control is that the angular velocity is equal to zero before falling perpendicularly on the lunar.The attitude control system of the lunar lander has the characteristics of complex nonlinearity, disturbance and strong coupling. For the problem, we do some researches in the following aspects:(1) Build the kinematic model of the attitude system for lunar lander by the order of 1-2-3 circumrotation.(2) Variable structure control with sliding mode was paid most attention because of its excellent invariance to the change of system parameters and external disturbances. However, it has its own disadvantage that after approaching the sliding surface, the system state will produce high frequency vibration around the sliding surface. Firstly, taking into account the vibration, utilize the sliding control having exponential convergent ability to design a nonlinear feedback controller. Secondly, develop a quasi sliding control based on the arc tangent function which can guarantee the system to have not only the better stability but also the well state response.(3) Solve the attitude maneuver of the normal attitude system based on the bang-bang control method, and use the nonlinear feedback controller to realize the attitude control based on Lyapunov theory since the stability isn't considered for the time optimal control.(4) Based on the above methods, the simulation results for an application are given. Comparing with the results, we find that the controller derived from the quasi sliding model control can eliminate the vibration, but its magnitude is much larger. The controller derived from combining the time optimal control and nonlinear feedback control not only has suitable magnitude but also is effective to deal with disturbances.